The Mickelson and Chen U.S. Pat. No. 4,335,266, reissued as Re 31,968 discloses methods of making graded I-III-VI.sub.2 semiconductors having internal p-n type regions formed by controlling the evaporation of the elements in the ternary compound so as to form a graded resistivity caused by a graded composition in the semiconductor. For example, a graded CuInSe.sub.2 thin film semiconductor could be formed with two distinct regions: a copper-rich p-type region near a base contact, and a copper-deficient n-type region near the photoactive junction of the thin film. Photocells produced by Mickelsen and Chen have photovoltaic efficiencies near 10%. This patent also suggests that it would be suitable to use the invention disclosed therein in conjunction with the quaternary compounds CuIn.sub.(1-x) Ga.sub.x Se.sub.2 or CuIn.sub.(1-x) Ga.sub.x S.sub.2 where x&lt;1.
CuInGaSe.sub.2 is a quaternary analog of the ternary compound CuInSe.sub.2 in which the Gallium is substituted on some Indium sites and serves to raise the band gap of the absorber to e.g. 1.15 eV for a Ga:In atomic ratio of 0.26:0.74.
In Devaney U.S. Pat. No. 4,684,761, a method is described which provides closer control of the elemental evaporation rates, and the substrate temperature particularly in the regions of the I-III-VI.sub.2 semiconductor near the contact and near the photoactive junction. This method provides photovoltaic cells having efficiencies of up to 11.9% AM1 (10.4% AMO) for CuInSe.sub.2 /CdZnS on alumina.
In our pending application Ser. No. 189,784 filed on May 4, 1988, higher efficiencies are described as resulting from the replacement of the ternary compound CuInSe.sub.2 with the quaternary analog CuInGaSe.sub.2. The replacement of the ternary compound CuInSe.sub.2 with the quaternary compound CuInGaSe.sub.2 results in a shift of the absorber band gap to higher energies. This replacement theoretically has an increased device voltage and therefore an increased photovoltaic efficiency.
The higher efficiencies that are theoretically possible with a quaternary compound CuInGaSe.sub.2 device have not been achieved for reasons that we believe are due mainly to the high optical absorption of the top window semiconductor layer which typically consists of CdS or CdZnS. The absorption is due in large part to the fundamental absorption edge in the window layer. Prior art cells do not have a window layer which effectively eliminates or significantly reduces window layer absorption.
Zinc oxide is a material which, when used as a window layer material, has shown reduced fundamental absorption losses in the visible and ultraviolet (UV) portions of the spectrum. Unfortunately, CuInSe.sub.2 /ZnO devices such as are disclosed in the Wieting et al U.S. Pat. No. 4,612,411, result in a somewhat reduced performance that is believed to be due to the leaky junction created between the ZnO layer and the CuInSe.sub.2 layer. Additionally, low resistivity ZnO layers have a limited effectiveness due to absorption of the infrared (IR) part of the light spectrum, also referred to as free carrier absorption. Finally, ZnO alone does not appear to form a heterojunction with CuInGaSe.sub.2.
Choudary et al. U.S. Pat. No. 4,611,091, describes high efficiency CuInSe.sub.2 /CdS/ZnO devices utilizing thin CdS layers between CuInSe.sub.2 and ZnO. Such devices achieve improved blue light response but show a degraded IR response.